Field of the Invention
The invention relates to a method of testing the leakage current at a contact-making point in an integrated circuit. The term "leakage current" is understood herein as meaning a current which flows between the contact-making point and supply potentials of the integrated circuit, when a potential is applied to the contact-making point.
One method of testing the leakage current at a contact-making point includes connecting the contact-making point to a constant voltage source and measuring the resultant leakage current directly. That requires an appropriate current measuring device.
Summary of the Invention
It is accordingly an object of the invention to provide a method of testing the leakage current at a contact-making point in an integrated circuit, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type and which can be carried out without a current measuring device and with little outlay for hardware.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of testing leakage current at a contact-making point in an integrated circuit, which comprises applying a test potential to the contact-making point through an output of an application device; connecting the output of the application device to a high impedance or isolating the output of the application device from the contact-making point; and determining a potential at the contact-making point as a measure of the leakage current produced.
Accordingly, with the invention, there is no direct measurement of the leakage current at the contact-making point. Therefore, no current measuring device is required for measuring the leakage current. Instead, the invention involves investigating how the potential at the contact-making point behaves after the test potential has been applied. The potential difference between the test potential and the determined potential is a measure of the electric charge flowing from the contact-making point and thus of the leakage current at the contact-making point. Since the output of the application device is connected to a high impedance, no charge can flow from the contact-making point to the application device or in the opposite direction during the predetermined time interval. Therefore, it is not possible for the application device to influence the leakage current.
In accordance with another mode of the invention, the method of testing the leakage current is initially carried out using a test potential at a first potential level and subsequently using a test potential at a second potential level. This affords the advantage of determining the various resultant leakage currents for various potential states of the contact-making point. This enables the respective circuit leakage current flowing from the contact-making point to the respectively opposite supply potential to be determined, particularly if the two potential levels are supply potential levels in the integrated circuit.
In accordance with a further mode of the invention, the potential at the contact-making point is determined and checked after a predetermined time interval to determine whether the potential determined has exceeded or fallen below a limit value. This predetermines a maximum permissible leakage current by predetermining a time interval and a limit value. In contrast to direct measurement of the potential, this embodiment has the advantage of permitting relatively simple technical measures to be used to monitor the limit value.
In accordance with an added mode of the invention, the method involves determining the time interval after which the potential at the contact-making point exceeds or falls below a limit value after the contact-making point has been subjected to the test potential. This requires a timer device which determines the time interval. In this case, the leakage current produced can be calculated as the product of the charge difference between the test potential and the determined potential at the contact-making point and the measured time interval.
In accordance with an additional mode of the invention, the potential at the contact-making point is determined by a determination unit which is a component of the integrated circuit. This has the advantage that no external circuit unit is required to determine the potential.
In accordance with yet another mode of the invention, the potential at the contact-making point is determined particularly simply by using an input circuit in the integrated circuit as a determination unit with a switching threshold. This input circuit is used to determine whether the potential at the contact-making point is above or below its switching threshold. This has the advantage of determining the potential by using a very simple component which can be used to supply external signals during normal operation of the circuit.
In accordance with yet a further mode of the invention, the test method is carried out by using an application device which is a component of the integrated circuit. No external application device is then required for applying the test potential to the contact-making point, so that the leakage current can be tested as part of a self-test in the integrated circuit.
In accordance with a concomitant mode of the invention, the time interval after which the potential at the contact-making point is determined or after which the potential has fallen below or exceeded the limit value is determined through the use of a timer device which is a component of the integrated circuit. This means that there is no need to supply the integrated circuit with a corresponding external time signal or to provide a further external connection that is necessary for this purpose.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of testing the leakage current at a contact-making point in an integrated circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.